Solid-state image pickup device, method for transferring charge in solid-state imaging device and method for manufacturing solid-state imaging device

ABSTRACT

The present invention solves a smear problem caused by mixing of a noise signal with signal electric charge being transferred in an operation to transfer the signal charge obtained as a result of a process carried out on a received light beam having a large wavelength. In order to solve the problem, the present invention provides a solid-state image pickup device including a layered structure which includes photosensors and an electric-charge transfer section. The photosensors include a first photosensor ( 21 ) and a second photosensor ( 22 ) for receiving a light beam with a wavelength smaller than the wavelength of a light beam received by the first photosensor ( 21 ). The first photosensor ( 21 ) and the second photosensor ( 2 ) are provided at adjacent locations separated away from each other by a potential barrier section ( 12 ). A read gate ( 42 ) provided beneath the first photosensor ( 21 ) transports electric charge obtained as a result of a process carried out by the first photosensor ( 21 ) to an electric-charge transfer section ( 50 ) provided beneath the second photosensor ( 22 ).

TECHNICAL FIELD

The present invention relates to a solid-state image pickup device andan electric-charge transfer method adopted by the device. To put it indetail, the present invention relates to a solid-state image pickupdevice capable of reducing smears generated in a red-color sensorportion and an electric-charge transfer method adopted by the device.

BACKGROUND ART

A recent solid-state image pickup device utilizing a semiconductor hasits chip area reduced by shrinking the size of a pixel device. As aresult, the cost per chip can also be lowered as well. In addition, thesize of an image pickup apparatus (or a camera system) itself can alsobe decreased.

The image pickup device includes mainly a photoelectric conversiondevice for converting a light beam coming from a lens into a signalelectric charge and a transfer CCD (Charge Couple Device) for deliveringthe signal electric charge to an amplifier for converting the signalelectric charge into an output voltage. In a typical configuration of apixel cell of an image pickup device, a photoelectric conversion deviceand a transfer CCD are laid out horizontally. With such a layout,however, it becomes practically difficult to shrink the size of theimage pickup device during a fabrication process. In order to solve thisproblem, a solid-state image pickup device has been proposed recently asa device with a layered structure in which a photoelectric conversiondevice is provided above a transfer CCD. For more information, refer todocuments such as pages 3 and 4 as well as FIG. 2 of Japanese PatentLaid-open No. 2001-257337.

In accordance with this conventional technology, however, transfer CCDsprovided below a photoelectric conversion device for RGB (Red, Green andBlue) pixels are not separated away from each other. For this reason, alight beam having a large wavelength as is the case with a red-colorlight beam for a red-color pixel of the RGB pixels penetrates thephotoelectric conversion device and hits the transfer CCDs to raise asmear problem caused by mixing of a noise signal with signal electriccharge being transferred. In order to solve this problem, a generationrate of smears for the depth of the semiconductor area of the transferCCD was examined with the depth of a photosensor taken as a parameter.Results of the examination are shown in FIGS. 9A and 9B. In FIGS. 9A and9B, the depth of a photosensor is taken as a parameter, the verticalaxes each represent the generation rate (or the cumulative generationrate) of smears and the horizontal axes each represent the depth of thesemiconductor area of the transfer CCD. To be more specific, FIG. 9Ashows examination results for a red-color light beam having a wavelengthof 700 nm. On the other hand, FIG. 9B shows examination results for agreen-color light beam having a wavelength of 550 nm. In the case of astructure with X=5 μm and ΔX=0.5 μm where notation X denotes the depthof the photosensor whereas notation ΔX denotes the depth of thesemiconductor area of the transfer CCD, for example, the generation rateof smears for a light beam with a wavelength of 700 nm (that is, for ared-color light beam) approaches about 4% as shown in FIG. 9A. For alight beam having a wavelength of 550 nm (that is, for a green-colorlight beam), on the other hand, the generation rate of smears is about1% as shown in FIG. 9B. As is obvious from the results of theexamination, the generation rate of smears for a red-color light beam isextremely high.

DISCLOSURE OF INVENTION

The present invention is a solid-state image-pickup device capable ofsolving the problems described above and an electric-charge transfermethod adopted by the device.

A first solid-state image pickup device provided by the presentinvention includes a solid-state image pickup device includingphotosensors and an electric-charge transfer section, which are providedin a layered structure. To be more specific, the photosensors include afirst photosensor and a second photosensor for receiving a light beamhaving a wavelength smaller than the wavelength of a light beam receivedby the first photosensor. The first and second photosensors are providedat adjacent locations separated away from each other by a potentialbarrier section. Below the first photosensor, a read gate is provided asa gate for transporting electric charge generated by the firstphotosensor to an electric-charge transfer section provided beneath thesecond photosensor.

To put in detail, in the first solid-state image pickup device citedabove, the first photosensor for receiving only a light beam having alarge wavelength converts the light beam having a large wavelength intoa signal electric charge in a photoelectric conversion process. The readgate transports the signal electric charge to the electric-chargetransfer section provided below the second photosensor for receiving alight beam having a wavelength smaller than the wavelength of the lightbeam received by the first photosensor. The electric-charge transfersection further transfers the signal electric charge transported by theread gate thereto. An example of a light beam having a large wavelengthis a red-color or green-color light beam and an example of a light beamhaving a small wavelength is a blue-color light beam. Thus, under thefirst photosensor for receiving a light beam having a large wavelength,no signal electric charge of the light beam having a large wavelength istransferred. As a result, even if a large-wavelength light beam such asa red-color light penetrates the first photosensor as is the case withthe conventional solid-state image pickup device, entering a spacebeneath the first photosensor, the light beam with the large wavelengthdoes not propagate to the electric-charge transfer section fortransferring the electric charge of the light beam with the largewavelength. For this reason, the first solid-state image pickup devicedoes not raise a smear problem caused by mixing of a noise signal withsignal electric charge in transferring the electric charge of anelectric beam having a large wavelength.

A second solid-state image pickup device provided by the presentinvention is a solid-state image pickup device including photosensorsand electric-charge transfer sections. The photosensors and theelectric-charge transfer sections are provided in a layered structure.To be more specific, the photosensors include a first photosensor and asecond photosensor for receiving a light beam having a wavelengthsmaller than the wavelength of a light beam received by the firstphotosensor. The first and second photosensors are provided at adjacentlocations separated away from each other by a potential barrier section.Below the first photosensor, a first electric-charge transfer section isprovided. In a side portion of the first photosensor, a first read gateis provided as a gate for transporting electric charge generated as aresult of a photoelectric conversion process carried out by the firstphotosensor to the first electric-charge transfer section. By the sametoken, below the second photosensor, a second electric-charge transfersection is provided and, in a side portion of the second photosensor, asecond read gate is provided as a gate for transporting electric chargegenerated as a result of a photoelectric-conversion process carried outby the second photosensor to the second electric-charge transfersection. A transfer gate is provided between the first electric-chargetransfer section and the second electric-charge transfer section as agate for transporting electric charge accumulated in the firstelectric-charge transfer section to the second electric-charge transfersection.

To put it in detail, in the second solid-state image pickup device citedabove, the first photosensor for receiving only a light beam having alarge wavelength converts the light beam having a large wavelength intoa signal electric charge in a photoelectric conversion process. Then,the first read gate transports the signal electric charge to the firstelectric-charge transfer section for accumulating the electric charge.With a predetermined timing, the transfer gate transports the electriccharge from the first electric-charge transfer section to the secondelectric-charge transfer section provided below the second photosensorfor receiving only a light beam having a small wavelength. The secondelectric-charge transfer section then transports the electric charge toa final stage. An example of a light beam having a large wavelength is ared-color or green-color light beam and an example of a light beamhaving a small wavelength is a blue-color light beam. On the other hand,the second photosensor for receiving only a light beam having awavelength smaller than the light beam received by the first photosensorconverts the light beam having a small wavelength into a signal electriccharge. With a timing of the second electric-charge transfer sectiontransferring no electric charge, the second read gate transports thesignal electric charge to the second electric-charge transfer section,which then transports the electric charge to typically the final stage.Thus, under the first photosensor for receiving a light beam having alarge wavelength, no signal electric charge of the light beam having alarge wavelength is transferred. As a result, even if a large-wavelengthlight beam such as a red-color (or green-color) light penetrates thefirst photosensor as is the case with the conventional solid-state imagepickup device, entering a space beneath the first photosensor, the lightbeam with the large wavelength does not propagate to the electric-chargetransfer section. For this reason, the second solid-state image pickupdevice does not raise a smear problem caused by mixing of a noise signalwith a signal electric charge being transferred.

A first electric-charge transfer-method provided by the presentinvention is provided for a solid-state image pickup device including afirst photosensor and a second photosensor for receiving a light beamhaving a wavelength smaller than the wavelength of a light beam receivedby the first photosensor. The first and second photosensors are providedat adjacent locations separated away from each other by a potentialbarrier section. Below the first photosensor, a read gate is provided asa gate for transporting electric charge generated by the firstphotosensor to an electric-charge transfer section provided beneath thesecond photosensor. In accordance with the first electric-chargetransfer method, the read gate transports an electric charge generatedas a result of a photoelectric conversion process carried out in thefirst photosensor to the electric-charge transfer section, which furthertransfers the electric charge.

To put it in detail, in accordance with the first electric-chargetransfer method described above, the first photosensor for receivingonly a light beam having a large wavelength converts the light beamhaving a large wavelength into an electric charge in a photoelectricconversion process. The read gate transports the electric charge to theelectric-charge transfer section provided below the second photosensorfor receiving a light beam having a wavelength smaller than thewavelength of a light beam received by the first photosensor. Theelectric-charge transfer section further transfers the electric chargetransported by the read gate thereto. An example of a light beam havinga large wavelength is a red-color or green-color light beam. Thus, underthe first photosensor for receiving a light beam having a largewavelength, no electric charge of the light beam having a largewavelength is transferred. As a result, even if a large-wavelength lightbeam such as a red-color light penetrates the first photosensor as isthe case with the conventional solid-state image pickup device, enteringa space beneath the first photosensor, the light beam with the largewavelength does not propagate to the electric-charge transfer sectionfor transferring the electric charge of the light beam with the largewavelength. For this reason, the first electric-charge transfer methoddoes not raise a smear problem caused by mixing of a noise signal withelectric charge in transferring the electric charge of an electric beamhaving a large wavelength.

A second electric-charge transfer method provided by the presentinvention is provided for a solid-state image pickup device including afirst photosensor and a second photosensor for receiving a light beamhaving a wavelength smaller than the wavelength of a light beam receivedby the first photosensor. The first and second photosensors are providedat adjacent locations separated away from each other by a potentialbarrier section. The solid-state image pickup device also includes afirst electric-charge transfer section provided below the firstphotosensor and a first read gate provided in a side portion of thefirst photosensor as a gate for transporting electric charge generatedas a result of a photoelectric conversion process carried out by thefirst photosensor to the first electric-charge transfer section. Inaddition, the solid-state image pickup device also has a secondelectric-charge transfer section provided below the second photosensorand a second read gate provided in a side portion of the secondphotosensor as a gate for transporting electric charge generated as aresult of a photoelectric conversion process carried out by the secondphotosensor to the second electric-charge transfer section. Thesolid-state image pickup device further includes a transfer gateprovided between the first electric-charge transfer section and thesecond electric-charge transfer section as a gate for transportingelectric charge accumulated in the first electric-charge transfersection to the second electric-charge transfer section. In accordancewith the second electric-charge transfer method, the first read gatetransports an electric charge generated as a result of a photoelectricconversion process carried out in the first photosensor to the firstelectric-charge transfer section. Furthermore, the transfer gatetransports the electric charge to the second electric-charge transfersection, which further transfers the electric charge.

To put in detail, in accordance with the second electric-charge transfermethod described above, the first photosensor for receiving only a lightbeam having a large wavelength converts the light beam having a largewavelength into a signal electric charge in a photoelectric conversionprocess. Then, the first read gate transports the signal electric chargeto the first electric-charge transfer section for accumulating theelectric charge. With a predetermined timing, the transfer gatetransports the electric charge from the first electric-charge transfersection to the second electric-charge transfer section. The secondelectric-charge transfer section then transports the electric charge toa final stage. An example of a light beam having a large wavelength is ared-color or green-color light beam. On the other hand, the secondphotosensor for receiving only a light beam having a wavelength smallerthan the light beam received by the first photosensor converts the lightbeam having a small wavelength into a signal electric charge. With atiming of the second electric-charge transfer section transferring noelectric charge, the second read gate transports the signal electriccharge to the second electric-charge transfer section, which thentransports the electric charge to typically the final stage. Thus, underthe first photosensor for receiving a light beam having a largewavelength, no signal electric charge of the light beam having a largewavelength is transferred. As a result, even if a large-wavelength lightbeam such as a red-color (or green-color) light penetrates the firstphotosensor as is the case with the conventional solid-state imagepickup device, entering a space beneath the first photosensor, the lightbeam with the large wavelength does not propagate to the electric-chargetransfer section. For this reason, the second electric-charge transfermethod does not raise a smear problem caused by mixing of a noise signalwith a signal electric charge being transferred.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B are diagrams showing embodiments implementing a firstsolid-state image pickup device and its first electric-charge transfermethod, which are provided by the present invention and, to be morespecific, FIG. 1A is a diagram showing a top-view layout whereas FIG. 1Bis a diagram showing a schematic cross section of a configuration;

FIGS. 2A and 2B are diagrams showing a transfer gate electrode and readgate electrode of the first solid-state image pickup device and, to bemore specific, FIG. 2A is a diagram showing a top-view layout whereasFIG. 2B is a diagram showing a schematic cross section of aconfiguration;

FIG. 3 shows timing charts of typical read operations carried out by thesolid-state image pickup device;

FIG. 4 is a diagram showing a schematic cross section of a configurationof embodiments implementing a second solid-state image pickup device andits second electric-charge transfer method, which are provided by thepresent invention;

FIGS. 5A and 5B are diagrams showing a transfer gate electrode and readgate electrode of the second solid-state image pickup device and, to bemore specific, FIG. 5A is a diagram showing a top-view layout whereasFIG. 5B is a diagram showing a schematic cross section of aconfiguration;

FIG. 6 shows timing charts of typical read operations carried out by thesecond solid-state image pickup device;

FIGS. 7A to 7C are diagrams showing a schematic top-view layout of thesecond electric-charge transfer method provided for a solid-state imagepickup device in accordance with the present invention;

FIGS. 8A and 8B are diagrams showing schematic cross sections of atypical method to fabricate the first solid-state image pickup device;and

FIGS. 9A and 9B are diagrams each showing a relation between the depthof a semiconductor area of a transfer CCD and the generation rate ofsmears with the depth of a photosensor taken as a parameter.

BEST MODES FOR CARRYING OUT THE INVENTION

By referring to FIG. 1A showing a top-view layout and FIG. 1B showing aschematic cross section of a configuration, the following descriptionexplains embodiments implementing a first solid-state image-pickupdevice and its first electric-charge transfer method, which are providedby the present invention.

As shown in FIG. 1A, in a typical configuration of the solid-state imagepickup device provided by the present invention, first photosensors 21(red-color photosensors 21R) each used for receiving a red-color lightbeam and first photosensors 21 (green-color photosensors 21G) each usedfor receiving a green-color light beam are laid out alternatelytypically in the vertical transfer direction. In addition, a secondphotosensor 22 (a blue-color photosensor 22B) for receiving a blue-colorlight beam is provided at a location adjacent to each pair of ared-color photosensor 21R and a green-color photosensor 21G. Suchblue-color photosensors 22B are laid out to form an array orientedtypically also in the vertical transfer direction.

The basic configuration of an electric-charge transfer method adopted bythe solid-state image pickup device described above is explained asfollows. In accordance with the electric-charge transfer method providedby the present invention, a signal electric charge obtained as a resultof a photoelectric conversion process carried out by the red-colorelectric-charge conversion section 21R (or the green-color photosensor21G) for receiving a light beam having a large wavelength and carryingout the photoelectric conversion process on the received light beam istransported to an electric-charge transfer section (or a transfer CCD)provided under the blue-color photosensor 22B for carrying out aphotoelectric conversion process to convert a light beam having awavelength smaller than the wavelength of the red or green light beam,and the electric-charge transfer section (or a transfer CCD) providedunder the blue-color photosensor 22B further transfers the signalelectric charge to an amplifier not shown in the figure. Examples of thelight beam having a large wavelength are a red-color light beam and, insome cases, a green-color light beam whereas an example of the lightbeam having a smaller wavelength is a blue-color light beam. Thered-color, green-color and blue-color light beams are the RGB (R: red,G: green and B: blue) light beams cited before. Arrows shown in thefigure each indicate a transport or transfer direction.

In a solid-state image pickup device, normally, the red-color light beam(and, in some cases, the green-color light beam) having a relativelylarge wavelength among the RGB (R: red, G: green and B: blue) lightbeams all but completely penetrates a photosensor made of asemiconductor layer. Thus, in the implementation of the basicconfiguration of the electric-charge transfer method provided for thesolid-state image pickup device, the electric-charge transfer section isnot provided under a first photosensor 21 (that is, the red-colorphotosensor 21R or the green-color photosensor 21G) for receiving ared-color light beam or, in some cases, a green-color light beam.Instead, a read gate is provided under the first photoelectric chargeconversion section 21 (21R, 21G) for receiving a light beam having alarge wavelength and carrying out a photoelectric conversion process onthe received light beam as a gate for transporting a signal electriccharge to an electric-charge transfer section (or a transfer CCD)provided beneath the second photosensor 22 (or a blue-color photosensor22B) for carrying out a photoelectric conversion process on a light beamhaving a wavelength smaller than the wavelength of the light beam. Theread gate transports the signal electric charge generated as a result ofthe photoelectric conversion process carried out by the firstphotosensor 21R (or 21G) to the electric-charge transfer section, whichthen further transfers the signal electric charge to an amplifier notshown in the figure. Examples of the light beam having a largewavelength are a red-color light beam, whereas an example of the lightbeam having a smaller wavelength is a blue-color light beam.

The above electric-charge transportation and transfer operations can becarried out because a light beam having a small wavelength as is thecase with the blue-color light beam is all but completely subjected to aphotoelectric conversion process on the surface of a semiconductor sothat no such light beam reaches the electric-charge transfer section (ora transfer CCD) provided beneath the second photosensor 22 to beconverted into an electric charge in the electric-charge transfersection and, hence, no electric charge is generated in theelectric-charge transfer section. Thus, the signal electric chargeobtained as a result of the photoelectric conversion process carried outon the red-color light beam flows through the electric-charge transfersection provided beneath the second photosensor 22 for receiving a lightbeam having a small wavelength (the blue color) in a transfer to theamplifier not shown in the figure. As a result, the red-color light beamcan be prevented from penetrating the photoelectric conversion sectionand hitting the electric-charge transfer section to raise a smearproblem caused by mixing of a noise signal with signal electric chargebeing transferred.

By referring to a schematically cross-sectional configuration diagram ofFIG. 1B, the following description explains the configuration of anembodiment implementing the first solid-state image pickup deviceprovided by the present invention as a device for realizing the firstelectric-charge transfer method also provided by the present invention.As an example, the configuration of the embodiment is explained for ared-color photosensor and a blue-color photosensor. It is to be notedthat, by replacing the red-color photosensor with a green-colorphotosensor, the following explanation also holds true of an embodimentcomprising a green-color photosensor and a blue-color photosensor.

As shown in FIG. 1B, the basic configuration of the solid-state imagepickup device provided by the present invention includes a semiconductorlayer 11. The semiconductor layer 11 has a first photosensor (or ared-color photosensor 21R) for receiving a light beam LL having a largewavelength as is the case with typically a red-color light beam andcarrying out a photoelectric conversion process on the light beam LL aswell as a second photosensor 22 (or a blue-color photosensor 22B) forreceiving a light beam LS having a small wavelength as is the case withtypically a blue-color light beam and carrying out a photoelectricconversion process on the light beam LS. The first photosensor and thesecond photosensor 22 are provided at adjacent locations separated awayfrom each other by a potential barrier section 12. The semiconductorlayer 11 is typically made of an SOI (Silicon On Insulator) layer. Ared-color filter 71R is provided on the red-color photosensor 21R whilea green-color filter is provided on a green-color photosensor, which isnot shown in the figure. On the other hand, a blue-color filter 71B isprovided on the blue-color photosensor 22B.

As shown in the top-view layout diagram of FIG. 1A, for example, firstphotosensors 21 (or red-color photosensors 21R) each used for receivinga red-color light beam and first photosensors 21 (or green-colorphotosensors 21G) each used for receiving a green-color light beam arelaid out alternately typically in the vertical transfer direction. Inaddition, a second photosensor 22 (or a blue-color photosensor 22B) forreceiving a blue-color light beam is provided at a location adjacent toeach pair of a red-color photosensor 21R and a green-color photosensor21G. Such-blue-color photosensors 22B are laid out to form an arrayoriented in the vertical transfer direction.

The following description is focused on the red-color photosensor 21Rand the blue-color photosensor 22B. The red-color photosensor 21R isprovided deeply to the bottom (a depth level of (X+ΔX)) of thesemiconductor layer 11. Over the semiconductor layer 11, the blue-colorphotosensor 22B is provided at a location adjacent to the red-colorphotosensor 21R, being separated away from the red-color photosensor 21Rby the potential barrier section 12. Within the semiconductor layer 11beneath the blue-color photosensor 22B, a channel area 51 of theelectric-charge transfer section is provided in a space with a thicknessof ΔX between a depth level of X and the bottom of the semiconductorlayer 11, being separated away from the blue-color photosensor 22B bythe potential barrier section 12. Beneath an insulation film (or a gateinsulation film) 31 under the semiconductor layer 11, a read gate 42 isprovided at a location below the red-color photosensor 21R and atransfer gate 53 is provided at a location below the blue-colorphotosensor 22B. As shown in none of the figures, a photosensor forreceiving a green-color light beam can be provided in the sameconfiguration as the aforementioned red-color photosensor 21R forreceiving a red-color light beam. In addition, in a side portion of theblue-color photosensor 22B, a vertically oriented read gate 52 forreading out a blue-color light beam is provided, being separated awayfrom the blue-color photosensor 22B by the potential barrier section 12and a gate insulation film 35.

In the configuration described above, the read gate 42 exists rightunder the red-color photosensor 21R for receiving a red-color light beam(or the green-color photosensor 21G for receiving a green-color lightbeam), being separated away from the red-color photosensor 21R or thegreen-color photosensor 21G by the gate insulation film 31 and noelectric-charge transfer section (transfer CCD) is present under thered-color photosensor 21R. In the red-color photosensor 21R (or thegreen-color photosensor 21G), a photoelectric conversion area isextended to the bottom of the semiconductor layer 11. Thus, the readgate 42 existing in an embedded oxide film not shown in the figuretransports a signal electric charge obtained as a result of aphotoelectric conversion process carried out by the red-colorphotosensor 21R receiving a red-color light beam (or the green-colorphotosensor 21G receiving a green-color light beam) on the light beam tothe electric-charge transfer section (or the transfer CCD) 50 providedbelow the blue-color photosensor 22B in a direction indicated by anarrow C shown in the figure. Then, the electric-charge transfer section50 further transfers this signal electric charge to, for example, thelast stage.

On the other hand, the vertically oriented read gate 52 on the left edgeof the blue-color photosensor 22B for receiving a blue-color light beamtransports a signal electric charge generated in the blue-colorphotosensor 22B to the electric-charge transfer section (or the transferCCD) 50 in a direction indicated by an arrow D shown in the figure.Then, the electric-charge transfer section 50 further transfers thissignal electric charge to, for example, the last stage.

Next, the transfer gate electrode and read gate electrode of the firstsolid-state image pickup device are described in detail by referring toFIG. 2A showing a top-view layout and FIG. 2B showing a schematic crosssection of a configuration. It is to be noted that FIG. 2B is a diagrammainly showing an A-A cross section of the gate electrode shown in FIG.2A.

As shown in FIGS. 2A and 2B, on the semiconductor layer 11, which istypically an SOI layer, a red-color photosensor 21R, a green-colorphotosensor 21G and a blue-color photosensor 22B for receiving lightbeams of the RGB colors respectively are laid out at locations separatedaway from each other by a potential barrier section not shown in thefigure. That is to say, red-color photosensors 21R and green-colorphotosensors 21G are laid out alternately typically in the verticaltransfer direction. In addition, a blue-color photosensor 22B isprovided at a location adjacent to each pair of a red-color photosensor21R and a green-color photosensor 21G, being separated away from thepair in the horizontal transfer direction. Beneath the semiconductorlayer 11 in which the red-color photosensor 21R and the green-colorphotosensor 21G are provided, read gates 42 (φA, φB and so on) extendedin the vertical transfer direction are each provided, being separatedaway from the semiconductor layer 11 by an insulation film such as agate insulation film as a gate capable of reading a signal electriccharge from the red-color photosensor 21R and the green-colorphotosensor 21G and transporting the signal electric charge to theelectric-charge transfer section (or the transfer CCD). For the readgates 42, transfer gates 53 (φ1, φ2 and so on) are laid out in thehorizontal transfer direction perpendicular to the read gates φA, φB andso on. Below the blue-color photosensor 22B, the transfer gates φ1, φ2and so on are laid out continuously, being separated away from theblue-color photosensor 22B by the gate insulation film 31. Below theread gates φA, φB and so on at the red-color photosensor 21R and thegreen-color photosensor 21G, on the other hand, the transfer gates φ1,φ2 and so on are laid out continuously, being separated away from thered-color photosensor 21R and the green-color photosensor 21G by aninsulation film 37.

A typical operation to read out signal electric charge in thesolid-state image pickup device 1 including the configuration describedabove is explained as follows. By laying out the transfer gates φ1, φ2and so on perpendicularly to the read gates φA, φB and so on, anoperation to read out a signal electric charge for the blue color can becarried out. The electric-charge transfer section (or the transfer CCD)provided below the blue-color photosensor 22B carries out a transferoperation by changing a voltage applied to the transfer gates φ1, φ2 andso on laid out in the horizontal direction. Right below thesemiconductor layer 11 (typically an SOI layer) in which the red-colorphotosensor 21R and the green-color photosensor 21G are provided, theread gates φA, φB and so on are provided whereas the transfer gates φ1,φ2 and so on are laid out through the read gates φA, φB and so on. Thus,large-wavelength light beams such as light beams of the green and redcolors are not affected.

Next, a typical operation to read out signal electric charge in thefirst solid-state image pickup device including the configurationdescribed above is explained by referring to timing charts shown in FIG.3 as follows.

As shown in FIG. 3, the read gate φA is turned on. Then, in order toread out a green-color signal, for example, the transfer gates φ1 and φ3and so on not shown in the figure are also turned on. Thus, the readgate φA transports a green-color signal obtained as a result of aphotoelectric conversion process carried out by the green-colorphotosensor to the electric-charge transfer sections of the transfergates φ1 and φ3 and so on and, then, the transfer gates φ1 and φ3 and soon not shown in the figure further transfer the green-color signal to anamplifier also not shown in the figure. A red-color signal is read outafter a predetermined necessary period T has lapsed since the operationto read out the green-color signal to prevent the red-color signal frombeing included in the green-color signal. To put it concretely, when thetransfer gates φ2 and φ4 and so on not shown in the figure are alsoturned on, the read gate φA transports the red-color signal obtained asa result of a photoelectric conversion process carried out by thered-color photosensor to the electric-charge transfer sections of thetransfer gates φ2 and φ4 and so on not shown in the figure and, then,the transfer gates φ2 and φ4 and so on further transfer the red-colorsignal to an amplifier not shown in the figure.

By referring to FIG. 4 showing a schematic cross section of aconfiguration of embodiments implementing a second solid-state imagepickup device and a second electric-charge transfer method, which areprovided by the present invention, the following description explainsthe embodiments. The typical configuration shown in FIG. 4 is differentfrom the typical configuration shown in FIG. 1B in that, in theconfiguration shown in FIG. 4, a transfer CCD is provided beneath aphotoelectric conversion device for the red color (or the blue color).As an example, FIG. 4 is given as an explanatory diagram showing ared-color photosensor and a blue-color photosensor.

As shown in FIG. 4, the basic configuration of the second solid-stateimage pickup device provided by the present invention includes asemiconductor layer 11. The semiconductor layer 11 has a firstphotosensor 21 (a red-color photosensor 21R) for receiving a light beamLL having a large wavelength as is the case with typically a red-colorlight beam and carrying out a photoelectric conversion process on thelight beam LL as well as a second photosensor 22 (a blue-colorphotosensor 22B) for receiving a light beam LS having a small wavelengthas is the case with typically a blue-color light beam and carrying out aphotoelectric conversion process on the light beam LS. The firstphotosensor 21 and the second photosensor 22 are provided at adjacentlocations separated away from each other by a potential barrier section12. The semiconductor layer 11 is typically made of an SOI (Silicon OnInsulator) layer. A red-color filter 71R is provided on the red-colorphotosensor 21R while a green-color filter is provided on a green-colorphotosensor, which is not shown in the figure. On the other hand, ablue-color filter 71B is provided on the blue-color photosensor 22B.

As shown in the top-view layout diagram of FIG. 1A, for example, firstphotosensors 21 (or red-color photosensors 21R) each used for receivinga red-color light beam and first photosensors 21 (or green-colorphotosensors 21G) each used for receiving a green-color light beam arelaid out alternately typically in the vertical transfer direction. Inaddition, a second photosensor 22 (or a blue-color photosensor 22B) forreceiving a blue-color light beam is provided at a location adjacent toeach pair of a red-color photosensor 21R and a green-color photosensor21G. Such blue-color photosensors 22B are laid out to form an arrayoriented in the vertical transfer direction.

The following description is focused on the red-color photosensor 21Rand the blue-color photosensor 22B. The red-color photosensor 21R andthe blue-color photosensor 22B are provided over the semiconductor layer11 at locations separated away from each other by a potential barriersection 12. In addition, in the semiconductor layer 11 beneath thered-color photosensor 21R, a channel area 41 of an electric-chargehold/transfer section 40 is extended to the bottom of the semiconductorlayer 11, being separated away from the red-color photosensor 21R by thepotential barrier section 12. Furthermore, in the semiconductor layer 11beneath the blue-color photosensor 22B, a channel area 51 of anelectric-charge transfer section is extended to the bottom of thesemiconductor layer 11, being separated away from the blue-colorphotosensor 22B by the potential barrier section 12. The channel areas41 and 51 are also separated away from each other by the potentialbarrier section 12.

In a side portion of the red-color photosensor 21R, a verticallyoriented first read gate 45 for reading out a red-color light beam isprovided, being separated away from the red-color photosensor 21R by thepotential barrier section 12 and a gate insulation film 33. On the otherhand, in a side portion of the blue-color photosensor 22B, a verticallyoriented second read gate 55 for reading out a blue-color light beam isprovided, being separated away from the blue-color photosensor 22B bythe potential barrier section 12 and a gate insulation film 35.

Beneath a gate insulation film 31 under the semiconductor layer 11, ahold gate 43 is provided at a location below the red-color photosensor21R and a transfer gate 53 is provided at a location below theblue-color photosensor 22B. In addition, a transfer gate 47 is providedbetween the hold gate 43 and the transfer gate 53 as a gate fortransferring electric charge from the electric-charge hold/transfersection 40 to the electric-charge transfer section 50. As shown in noneof the figures, a photosensor for receiving a green-color light beam andits electric-charge hold/transfer section can be provided in the sameconfiguration as the aforementioned red-color photosensor 21R forreceiving a red-color light beam and the electric-charge hold/transfersection 40.

As explained above, in the configuration, the electric-chargehold/transfer section 40 (comprising the channel area 41, the gateinsulation film 31 and the hold gate 43) exists beneath the red-colorphotosensor 21R for receiving a red-color light beam (or the green-colorphotosensor 21G for receiving a green-color light beam), being separatedaway from the red-color photosensor 21R (or the green-color photosensor21G) by the potential barrier section 12. However, the electric-chargetransfer section (or the transfer CCD) for transferring electric chargein the vertical direction does not exist beneath the red-colorphotosensor 21R (or the green-color photosensor 21G). Thus, the firstread gate 45 transports a red-color signal (or a green-color signal)generated in a photoelectric conversion process carried out by thered-color photosensor 21R for receiving a red-color light beam (or thegreen-color photosensor 21G for receiving a green-color light beam) tothe electric-charge hold/transfer section 40 for temporarilyaccumulating the signal transported thereto. With a predeterminedtiming, the transfer gate 47 transports the signal electric chargeaccumulated in the electric-charge hold/transfer section 40 from theelectric-charge hold/transfer section 40 in a direction indicated by anarrow C shown in the figure to the electric-charge transfer section 50(comprising the channel area 51, the gate insulation film 31 and thetransfer gate 53). The electric-charge transfer section 50 furthertransfers the signal electric charge typically to the final stage.

On the other hand, the second read gate 55 on the left edge of theblue-color photosensor 22B for receiving a blue-color light beamtransports a signal electric charge generated by the blue-colorphotosensor 22B to the electric-charge transfer section 50 in adirection indicated by an arrow D shown in the figure. Theelectric-charge transfer section 50 further transfers the signalelectric charge typically to the final stage.

Next, the transfer gate electrode and read gate electrode of the secondsolid-state image pickup device are explained in detail by referring toFIG. 5A showing a top-view layout and FIG. 5B showing a schematic crosssection of a configuration. It is to be noted that FIG. 5B is a diagrammainly showing an A-A cross section of the gate electrode shown in FIG.5A.

As shown in FIGS. 5A and 5B, on the semiconductor layer 11, which istypically an SOI layer, a red-color photosensor 21R, a green-colorphotosensor 21G and a blue-color photosensor 22B for receiving lightbeams of the RGB colors respectively are laid out at locations separatedaway from each other by a potential barrier section not shown in thefigure. That is to say, red-color photosensors 21R and green-colorphotosensors 21G are laid out alternately typically in the verticaltransfer direction. In addition, a blue-color photosensor 22B isprovided at a location adjacent to each pair of a red-color photosensor21R and a green-color photosensor 21G, being separated away from thepair in the horizontal transfer direction.

Beneath the semiconductor layer 11 in which the red-color photosensor21R and the green-color photosensor 21G are provided, a hold gate 43(Vh) of the electric-charge hold/transfer section 40 for temporarilyholding an electric charge read out from the red-color photosensor 21Rand the green-color photosensor 21G is provided typically in thevertical transfer direction, being separated away from the semiconductorlayer 11 by an insulation film such as the gate insulation film. Inaddition, beneath the semiconductor layer 11 at locations among thered-color photosensor 21R, the green-color photosensor 21G and theblue-color photosensor 22B, transfer gates 47 (φa, φb and so on) areprovided in parallel to the hold gates Vh, being separated away from thesemiconductor layer 11 by an insulation film such as the gate insulationfilm. The transfer gates 47 (φa, φb and so on) serve as gates fortransferring electric charge from the electric-charge hold/transfersection 40 to the electric-charge transfer section 50 provided below theblue-color photosensor 22B. Furthermore, transfer gates 53 (φ1, φ2 andso on) are laid out in a horizontal transfer direction perpendicular tothe hold gates Vh and the transfer gates φa, φb and so on. The transfergates 53 (φ1, φ2 and so on) serve as gates for transferring the electriccharge typically to the final stage. Below the blue-color photosensor22B, the transfer gates φ1, φ2 and so on are laid out continuously,being separated away from the blue-color photosensor 22B by the gateinsulation film 31. At the red-color photosensor 21R and the green-colorphotosensor 21G, on the other hand, the transfer gates φ1, φ2 and so onare laid out continuously, being separated away from the red-colorphotosensor 21R and the green-color photosensor 21G by an insulationfilm 37 beneath the hold gates Vh and the transfer gates φa, φb and soon.

A typical operation to read out signal electric charge in thesolid-state image pickup device 2 including the configuration describedabove is explained as follows. By laying out the transfer gates φ1, φ2and so on perpendicularly to the hold gates Vh, Vh and so on, anoperation to read out a signal electric charge for the blue color can becarried out. The electric-charge transfer section (or the transfer CCD)50 provided below the blue-color photosensor 22B carries out a transferoperation by changing a voltage applied to the transfer gates φ1, φ2 andso on laid out in the horizontal direction. Right below thesemiconductor layer 11 (typically an SOI layer) in which the red-colorphotosensor 21R and the green-color photosensor 21G are provided, thehold gates Vh, Vh and so on as well as the transfer gates φa, φb and soon are provided, whereas the transfer gates φ1, φ2 and so on are laidout through the hold gate Vh and the transfer gate φa, the hold gate Vhand the transfer gate φb and so on. Thus, large-wavelength light beamssuch as light beams of the green and red colors are not affected.

Next, a typical operation to read out signal electric charge in thesecond solid-state image pickup device including the configurationdescribed above is explained by referring to timing charts shown in FIG.6 as follows.

As shown in FIG. 6, the transfer gate φa is turned on. Then, in order toread out a green-color signal, for example, the transfer gates φ1 and φ3and so on not shown in the figure are also turned on. Thus, the transfergates φa, φb and so on each transport a green-color signal obtained as aresult of a photoelectric conversion process carried out by thegreen-color photosensor and held in the electric-charge hold/transfersection to the electric-charge transfer sections of the transfer gatesφ1, φ3 and so on not shown in the figure and, then, the transfer gatesφ1, φ2, φ3 and so on not shown in the figure further transfer thegreen-color signal to an amplifier not shown in the figure. A red-colorsignal is read out after a predetermined necessary period T has lapsedsince the operation to read out the green-color signal to prevent thered-color signal from being included in the green-color signal. In orderto read out a red-color signal, the transfer gates φ2 and φ4 and so onnot shown in the figure are turned on. Then, the transfer gate φa isturned on to transport a red-color signal obtained as a result of aphotoelectric conversion process carried out by the red-colorphotosensor and held in the electric-charge hold/transfer section to theelectric-charge transfer sections of the transfer gates φ2, φ4 and so onnot shown in the figure and, then, the transfer gates φ1, φ2, φ3 and soon not shown in the figure further transfer the red-color signal to theamplifier not shown in the figure.

In the configuration of the embodiment described above, noise-signalelectric charge is generated in the electric-charge transfer section (orthe transfer CCD) due to smears. However, no noises enter the signalelectric charge, which is flowing through the electric-charge transfersection 50 for the blue color, from the electric-charge hold/transfersection 40 for the red color (or the green color) adjacent to theelectric-charge transfer section 50. Thus, the signal is notcontaminated.

Next, another electric-charge transfer method adopted by the secondsolid-state image pickup device provided by the present invention isexplained by referring to FIGS. 7A to 7C showing a schematic top-viewlayout of the electric-charge transfer method. It is to be noted thatarrows shown in the figures each indicate the direction of a transfer ofelectric charge.

The following description explains a case in which a signal electriccharge generated as a result of a photoelectric conversion processcarried out by the red-color photosensor is not capable of reaching theelectric-charge transfer section (or the transfer CCD) of the blue-colorarea unless the signal electric charge goes through the electric-chargetransfer section provided beneath the red-color photosensor. In thiscase, prior to a process to read out the signal electric charge of thered color, a cleaning operation is carried out in order to remove noiseelectric charge of the electric-charge transfer section for the redcolor. That is to say, as shown in FIG. 7A, the noise-signal electriccharge N is moved from the electric-charge transfer section (or thetransfer CCD) 50R for the red color to the electric-charge transfersection (or the transfer CCD) 50B of the blue-color area. Then, theelectric-charge transfer section (or the transfer CCD) 50B of thecontinuous blue-color area discards the noise-signal electric charge Nto the final stage. Operations are carried out for the signal electriccharge of the green color in the same way as the signal electric chargeof the red color. That is to say, noise-signal electric charge N ismoved from the electric-charge transfer section (or the transfer CCD)50G for the green color to the electric-charge transfer section (or thetransfer CCD) 50B of the blue-color area. Then, the electric-chargetransfer section (or the transfer CCD) 50B of the continuous blue-colorarea discards the noise-signal electric charge N to the final stage.

Subsequently, as shown in FIG. 7B, after the noise-signal electriccharge N is discarded, the read gate of the red color is turned onimmediately and red-color signal electric charge SR is moved from thered-color photosensor to the electric-charge transfer section 50R forthe red color. Then, the electric-charge transfer section 50R for thered color is turned on to move the red-color signal electric charge SRto the electric-charge transfer section (or the transfer CCD) 50B of theblue-color area. Subsequently, the electric-charge transfer section (orthe transfer CCD) 50B of the continuous blue-color transfers thered-color signal electric charge SR. Operations are carried out for thesignal electric charge of the green color in the same way as the signalelectric charge of the red color. That is to say, after the noise-signalelectric charge N is discarded, the read gate of the green color isturned on immediately and green-color signal electric charge is movedfrom the green-color photosensor to the electric-charge transfer section50G for the green color. Then, the electric-charge transfer section 50Gfor the green color is turned on to move the green-color signal electriccharge to the electric-charge transfer section (or the transfer CCD) 50Bof the blue-color area. Subsequently, the electric-charge transfersection (or the transfer CCD) 50B of the continuous blue-color transfersthe green-color signal electric charge.

It is to be noted that, if only signal electric charges of the red andgreen colors can be moved to the electric-charge transfer section 50B ofthe blue color, it is basically unnecessary to carry out the cleaningoperation described above. This is because, since noises accumulated inthe electric-charge transfer section 50R (or 50G) for the red color (orthe green color) are also originally generated from a light beam of thered (or green) color from the beginning, it is nice to merely considersignal electric charge comprising an electric charge obtained as aresult of a photoelectric conversion process carried out by thephotosensor and additional noise electric charge accumulated in theelectric-charge transfer section. Then, if this signal electric chargeis sent to the electric-charge transfer section (or the transfer CCD)50B for the blue color, the operations will become the same as thoseexplained in the description of the first embodiment.

By the way, as shown in FIG. 7C, after a green-color light beam has beensubjected to a photoelectric conversion process carried out by thegreen-color photosensor 50G to generate a green-color signal electriccharge SG, for example, the green-color signal electric charge SG issent to the electric-charge transfer section (or the transfer CCD) 50Bfor the blue color by way of the electric-charge transfer section (orthe transfer CCD) 50R for the red color in the following order: theelectric-charge transfer section (or the transfer CCD) 50G for the greencolor, the electric-charge transfer section (or the transfer CCD) 50Rfor the red color, the electric-charge transfer section (or the transferCCD) 50B for the blue color, the electric-charge transfer section (orthe transfer CCD) 50B for the blue color and so on. In this case, it isnecessary to once carry out a cleaning operation to remove noise chargefrom the electric-charge transfer section 50R for the red color beforesending the green-color signal electric charge SG. If the cleaningoperation is not carried out, the green-color signal will be mixed withthe red-color noise, getting contaminated.

As described above, when the electric-charge transfer section (or thetransfer CCD) for the red (or green) color is used deliberately, beforethe signal electric charge for the red color is read out, a cleaningoperation is carried out in order to remove noise charges from theelectric-charge transfer section for the red color. That is to say, thenoise-signal electric charge is moved from the electric-charge transfersection for the red color to the electric-charge transfer section forthe blue color and, then, the red color to the electric-charge transfersection for the blue color discards the noise-signal electric charge tothe final stage.

Next, a typical method of fabricating the first solid-state image pickupdevice is explained by referring to FIGS. 8A and 8B showing schematiccross sections of the method.

First of all, as shown in FIG. 8A, an SOI substrate 3 is prepared assubstrate A, and an element separation area not shown in the figure isprovided on its semiconductor layer (SOI layer) 4 by adopting a knownelement separation technology such as a trench element separationtechnology. In addition, if a bulk semiconductor substrate is used, forexample, it is nice to create a trench element separation area to thedepth corresponding to the film thickness of a desired SOI layer. It isto be noted that, if an SOI substrate is used and there is no need tocreate an element separation area, the element separation area is notprovided.

Then, for the semiconductor layer 4, an ion injection method istypically adopted to create, among other things, an embedded channel(such as an n-type semiconductor area) for a transfer gate and apotential barrier section (such as a p-type channel stop area). Theseembedded channel and the potential barrier section are not shown in thefigures.

Subsequently, by adoption of a known technology for creating a gateelectrode, a read gate (or a read gate electrode) 42 is provided on thesemiconductor layer 4, being separated away from the semiconductor layer4 by a gate insulation film 31. In this process, first of all, the gateinsulation film 31 is provided before a multi-crystal silicon film forthe gate electrode is formed. Then, a lithography technology using anordinary resist mask and an etching technology are adopted to create aread gate electrode 42G laid out in the vertical transfer direction.Subsequently, after unnecessary resist is removed, an insulation filmcovering the read gate electrode 42G is provided. Then, a multi-crystalsilicon film for the transfer gate electrode is formed. Subsequently,the lithography technology using an ordinary resist mask and the etchingtechnology are adopted to create a transfer gate electrode laid out inthe horizontal transfer direction. The transfer gate electrode is notshown in the figure. Then, unnecessary resist is removed. In addition,if a vertically oriented read gate (or a vertically oriented read gateelectrode) for reading out signal electric charge of the blue color isto be provided, prior to the formation of the gate electrode, a groovefor laying out the vertically oriented read gate electrode is providedon the semiconductor layer 4. Afterwards, by embedding multi-crystalsilicon in the groove, the vertically oriented read gate electrode forreading out signal electric charge of the blue color can be provided.The vertically oriented read gate electrode for reading out signalelectric charge of the blue color is also not shown in the figure.

Thereafter, an inter-layer insulation film 6 for covering the gateelectrodes is provided. Then, a CMP (Chemical Mechanical Polishing)technology is adopted to make the surface of the inter-layer insulationfilm 6 flat. The flattened surface is then stuck to other substrate B.Afterwards, an anneal process is carried out at a typical temperature of1,000° C. for about 1 hour to implement a dehydrogenation bindingprocess for firmly binding substrates A and B with each other.

Then, the back face of substrate A is ground. Later on, an etchingprocess is carried out. The etching process is ended on the basis of adifference in etching rate between an embedded oxide film 5 of the SOIsubstrate 3 and the silicon of substrate A. By doing so, as shown inFIG. 8B, the embedded oxide film 5 of the SOI substrate 3 is exposed.Next, an ion injection process is carried out in order to obtain adesired distribution of impurities for the semiconductor layer 4.

Then, by adoption of a known technology for creating an on-chip lens anda known technology for creating an on-chip color filter, pixels areprovided to satisfy specifications of the on-chip lens and the on-chipcolor filter. This process is not shown in the figure.

It is to be noted that a method of fabricating the second solid-stateimage pickup device is identical with the method of fabricating thefirst solid-state image pickup device. That is to say, in accordancewith the method of fabricating the second solid-state image pickupdevice, in the process to create a read gate in accordance with themethod of fabricating the first solid-state image pickup device, a holdgate and a transfer gate are also provided and, in the process to createa vertically oriented read gate in accordance with the method offabricating the first solid-state image pickup device, a verticallyoriented read gate for the second solid-state image pickup device isalso provided. The rest of the method of fabricating the secondsolid-state image pickup device is the same as the other processes ofthe method of fabricating the first solid-state image pickup device.

As described above, in accordance with the first solid-state imagepickup device provided by the present invention, a signal electriccharge of a light beam having a large wavelength as is the case with alight beam of the red color (or the green color) can be transported tothe electric-charge transfer section provided beneath the secondphotosensor for receiving a light beam having a small wavelength as isthe case with a light beam of the blue color by virtue of the read gateprovided beneath the first photosensor for receiving a light beam havinga large wavelength. In accordance with the second solid-state imagepickup device provided by the present invention, on the other hand,after accumulation of a signal electric charge of a light beam having alarge wavelength as is the case with a light beam of the red color (orthe green color), the signal electric charge can be transported to theelectric-charge transfer section provided beneath the second photosensorfor receiving a light beam having a small wavelength as is the case witha light beam of the blue color by virtue of the hold gate and thetransfer gate, which are provided beneath the first photosensor forreceiving a light beam having a large wavelength. Thus, even if a lightbeam having a large wavelength penetrates the first photosensor duringan operation to transfer the signal electric charge of a light beamhaving a large wavelength in either of the solid-state image pickupdevices as is the case with the conventional solid-state image pickupdevice, the transfer of the signal electric charge of a light beamhaving a large wavelength will no longer be affected. As a result, it ispossible to solve to a smear problem caused by mixing of a noise signalwith signal electric charge being transferred due to propagation of alight beam to the electric-charge transfer section.

In accordance with the first electric-charge transfer method providedfor a solid-state image pickup device in accordance with the presentinvention, a signal electric charge of a light beam having a largewavelength as is the case with a light beam of the red color (or thegreen color) can be transported to the electric-charge transfer sectionprovided beneath the second photosensor for receiving a light beamhaving a small wavelength as is the case with a light beam of the bluecolor by virtue of the read gate provided beneath the first photosensorfor receiving a light beam having a large wavelength. In accordance withthe second electric-charge transfer method provided for a solid-stateimage pickup device in accordance with the present invention, on theother hand, after accumulation of a signal electric charge of a lightbeam having a large wavelength as is the case with a light beam of thered color (or the green color), the signal electric charge can betransported to the electric-charge transfer section provided beneath thesecond photosensor for receiving a light beam having a small wavelengthas is the case with a light beam of the blue color by virtue of the holdgate and the transfer gate, which are provided beneath the firstphotosensor for receiving a light beam having a large wavelength. Thus,even if a light beam having a large wavelength penetrates the firstphotosensor during an operation to transfer the signal electric chargeof a light beam having a large wavelength by adoption of either of theelectric-charge transfer methods as is the case with the conventionalsolid-state image pickup device, the transfer of the signal electriccharge of a light beam having a large wavelength will no longer beaffected. As a result, it is possible to solve to a smear problem causedby mixing of a noise signal with signal electric charge beingtransferred due to propagation of a light beam to the electric-chargetransfer section.

1. A solid-state image pickup device comprising: a photosensor sectionprovided in a substrate as a section including a first photosensor and asecond photosensor for receiving a light beam with a wavelength smallerthan the wavelength of a light beam received by said first photosensor;an electric-charge transfer section provided beneath said secondphotosensor in said substrate; and a read gate provided beneath saidfirst photosensor in said substrate as a gate for transporting electriccharge obtained as a result of a photoelectric conversion processcarried out by said first photosensor to said electric-charge transfersection.
 2. A solid-state image pickup device according to claim 1,wherein said first photosensor receives a light beam of the red or greencolor and said second photosensor receives a light beam of the bluecolor.
 3. A solid-state image pickup device according to claim 1,wherein said first photosensor receives a light beam of the red colorand said second photosensor receives a light beam of the green color. 4.A solid-state image pickup device according to claim 1, wherein saidfirst photosensor and said second photosensor are provided at adjacentlocations separated away from each other by a potential barrier section.5. A solid-state image pickup device comprising: a photosensor sectionprovided in a substrate as a section including a first photosensor and asecond photosensor for receiving a light beam with a wavelength smallerthan the wavelength of a light beam received by said first photosensor;a first electric-charge transfer section provided beneath said firstphotosensor in said substrate; a second electric-charge transfer sectionprovided beneath said second photosensor in said substrate; a first readgate provided in a side portion of said first photosensor in saidsubstrate as a gate for transporting electric charge obtained as aresult of a photoelectric conversion process carried out by said firstphotosensor to said first electric-charge transfer section; a secondread gate provided in a side portion of said second photosensor in saidsubstrate as a gate for transporting electric charge obtained as aresult of a photoelectric conversion process carried out by said secondphotosensor to said second electric-charge transfer section; and atransfer gate provided between said first electric-charge transfersection and said second electric-charge transfer section in saidsubstrate as a gate for transferring electric charge accumulated in saidfirst electric-charge transfer section to said second electric-chargetransfer section.
 6. A solid-state image pickup device according toclaim 5, wherein said first photosensor receives a light beam of the redor green color and said second photosensor receives a light beam of theblue color.
 7. A solid-state image pickup device according to claim 5,wherein said first photosensor receives a light beam of the red colorand said second photosensor receives a light beam of the green color. 8.A solid-state image pickup device according to claim 5, wherein saidfirst photosensor and said second photosensor are provided at adjacentlocations separated away from each other by a potential barrier section.9. An electric-charge transfer method provided for a solid-state imagepickup device comprising a photosensor section provided in a substrateas a section including a first photosensor and a second photosensor forreceiving a light beam with a wavelength smaller than the wavelength ofa light beam received by said first photosensor, wherein a read gateprovided beneath said first photosensor in said substrate transportselectric charge obtained as a result of a photoelectric conversionprocess carried out by said first photosensor to an electric-chargetransfer section provided beneath said second photosensor in saidsubstrate.
 10. An electric-charge transfer method provided for asolid-state image pickup device in accordance with claim 9, wherein saidfirst photosensor receives a light beam of the red or green color andsaid second photosensor receives a light beam of the blue color.
 11. Anelectric-charge transfer method provided for a solid-state image pickupdevice in accordance with claim 9, wherein said first photosensorreceives a light beam of the red color and said second photosensorreceives a light beam of the green color.
 12. An electric-chargetransfer method provided for a solid-state image pickup device inaccordance with claim 9, wherein said first photosensor and said secondphotosensor are provided at adjacent locations separated away from eachother by a potential barrier section.
 13. An electric-charge transfermethod provided for a solid-state image pickup device comprising aphotosensor section provided in a substrate as a section including afirst photosensor and a second photosensor for receiving a light beamwith a wavelength smaller than the wavelength of a light beam receivedby said first photosensor, wherein: a read gate provided in a sideportion of said first photosensor in said substrate transports electriccharge obtained as a result of a photoelectric conversion processcarried out by said first photosensor to a first electric-chargetransfer section provided beneath said first photosensor in saidsubstrate; and a transfer gate further transfers said electric charge toa second electric-charge transfer section provided beneath said secondphotosensor in said substrate.
 14. An electric-charge transfer methodprovided for a solid-state image pickup device in accordance with claim13, wherein said first photosensor receives a light beam of the red orgreen color and said second photosensor receives a light beam of theblue color.
 15. An electric-charge transfer method provided for asolid-state image pickup device in accordance with claim 13, whereinsaid first photosensor receives a light beam of the red color and saidsecond photosensor receives a light beam of the green color.
 16. Anelectric-charge transfer method provided for a solid-state image pickupdevice in accordance with claim 13, wherein said first photosensor andsaid second photosensor are provided at adjacent locations separatedaway from each other by a potential barrier section.
 17. A method offabricating a solid-state image pickup device, said method comprisingthe steps of creating: a photosensor section in a substrate as a sectionincluding a first photosensor and a second photosensor for receiving alight beam with a wavelength smaller than the wavelength of a light beamreceived by said first photosensor; an electric-charge transfer sectionbeneath said second photosensor in said substrate; and a read gatebeneath said first photosensor in said substrate as a gate fortransporting electric charge obtained as a result of a photoelectricconversion process carried out by said first photosensor to saidelectric-charge transfer section.
 18. A method of fabricating asolid-state image pickup device, said method comprising the steps ofcreating: a photosensor section in a substrate as a section including afirst photosensor and a second photosensor for receiving a light beamwith a wavelength smaller than the wavelength of a light beam receivedby said first photosensor; a first electric-charge transfer sectionbeneath said first photosensor in said substrate; a secondelectric-charge transfer section beneath said second photosensor in saidsubstrate; a first read gate provided in a side portion of said firstphotosensor in said substrate as a gate for transporting electric chargeobtained as a result of a photoelectric conversion process carried outby said first photosensor to said first electric-charge transfersection; a second read gate provided in a side portion of said secondphotosensor in said substrate as a gate for transporting electric chargeobtained as a result of a photoelectric conversion process carried outby said second photosensor to said second electric-charge transfersection; and a transfer gate between said first electric-charge transfersection and said second electric-charge transfer section in saidsubstrate as a gate for transferring electric charge accumulated in saidfirst electric-charge transfer section to said second electric-chargetransfer section.